In recent years, one of the trends in inkjet technology is that colorants in inkjet inks have advanced from dye molecules to pigment particles. Compared to dye-based ink, pigment-based colorants provided much better image permanence. For example, light-fade or ozone fade of an image printed with pigment-based inks is much slower than that of an image printed with dye-based inks. At the same time, water resistance is also significantly improved because dye molecules are more readily dissolved into water than are pigment particles. Therefore, dye-based images are easily damaged by water splashing, rain, or water droplets.
Some challenges exist with respect to pigmented inks, however. When pigment based inks are printed on traditional glossy coated specialty media such as glossy brochure paper or photo papers, some image quality and durability problems are encountered. In addition to the common image defects like feathering and coalescence, another common defect is print mottle. Mottling often presents as uneven random color patterns in a large area of an image. It is generally believed that uneven absorption of ink vehicle in the coating layer causes this defect, a result of uneven coat weight/thickness on base paper, and/or variation of pore structure in the coating layer. For coated paper, the underneath base paper is usually rougher than the final sheets. During a typical coating process, the thickness of the coating layer may vary with any bumps and valleys on the base paper surface. For media products produced with blade or air knife or similar technology, the coat weight variation could be appreciable, depending to a great extent on the surface roughness of the base paper. Even with precise coating methods such as curtain coating, in which the coat weigh uniformity is less dependent upon the topography of the base paper, there is often uneven coating thickness across the web. Since the absorption of liquid in coating layer is different than absorption in the base paper, variation of the coat weight is a major cause of print mottle. On another hand, coated paper usually goes through a calender or super calender step after the coating process in order to produce high glossy products. Under pressure and/or high temperature, the pores in the coating layer will deform. Due to uneven base paper and variation of coating thickness, calendering can easily cause differences in pore structure, i.e., patterns of pore size distribution and pore shape. Such differences will in many cases cause variation of ink penetration rate in the coating layer, and eventually exacerbate a print mottle defect.
In addition to print mottle defects, image permanence is another major challenge associated with the use of pigment-based inkjet inks on glossy coated media. After printing, wet pigment-based inkjet images on printed sheets are generally susceptible to smearing under any one of a number of conditions, such as rubbing with dry or wet fingers, rubbing with other printed or blank sheets, and marking with high-lighter pen. High-lighter smear is a major concern, especially on glossy media, where there are no small pores or recesses on a smooth surface for pigments to anchor. If pigment adhesion to the media surface is not sufficiently strong, ink particles are easily smeared with the rubbing of a high-lighter fluid. Hence, a matte media with the same coating tends to give better high-lighter smear resistance than a glossy product.
With recent advances in technology, inkjet technology has broadened its application to commercial and industrial printing, in addition to home and office usage. Following this trend, high-speed printing is required to support many new applications for inkjet technology. Glossy or matte porous specialty media are some of the choices in the media portfolio for these new high-speed inkjet printing applications. Traditional inkjet specialty media tend to be unsatisfactory for use with the new high-speed printers. In part, this is because of the large amount of relatively expensive small particle size materials, such as fumed silica, silica gel, fumed or colloidal alumina, boehmite, or mixtures of those, which are customarily used in the media in order to obtain good image quality and print quality. Although some available inkjet specialty media containing those materials may give excellent image quality and image permanence, their complicated manufacturing requirements and relatively high cost make the new high-speed inkjet printing systems noncompetitive with existing analogue printing methods and laserjet technology.